1. Field of the Invention
This invention relates to a fill suitable for use with tungsten halogen lamps and lamps using same. More particularly, this invention relates to a tungsten halogen lamp operating at a temperature greater than 250.degree. C. and having improved life which comprises a hermetically sealed, light transmissive, vitreous envelope containing a tungsten filament and a mixture of inert gas, hydrogen, phosphorus and bromine wherein atomic ratio of phosphorus to bromine ranges from 0.4-2.5.
2. Background of the Disclosure
Tungsten halogen lamps which comprise a hermetically sealed vitreous, light transmissive envelope enclosing a tungsten filament and containing a fill comprising a halogen gas are well known to those skilled in the art. These lamps operate on a tungsten-halogen cycle which is a regenerative, continuous process in which a halogen containing tungsten compound is produced when the halide combines chemically with particles of tungsten evaporating from an incandescent tungsten filament. Subsequent thermal decomposition of these so formed halogen containing tungsten compounds at the filament returns the tungsten particles back to the filament. For the most part, one or more bromine compounds are used for the halide fill in such lamps along with an inert gas, such as argon. Phosphorus or phosphorus compounds which act as oxygen getters are also employed in the fill. Bromine compounds known to be useful for such fills include hydrogen bromide HBr, methyl bromide CH.sub.3 Br, dibromomethane CH.sub.2 Br.sub.2 and bromoform CHBr.sub.3. Thus, U.S. Pat. No. 3,712,701 discloses the use of hydrogen bromide as well as these bromine containing single carbon atom hydrocarbon compounds. This patent also discloses the use of phosphonitrilic bromides. U.S. Pat. No. 4,629,935 discloses a fill consisting of a mixture of phosphine and methyl bromide wherein the mass ratio of the carbon to the phosphorus in the fill resulting from such compounds ranges from about 4/1 to about 1/1. It is also known that the use of carbon containing compounds in tungsten-halogen lamps can result in the tungsten filament becoming brittle due to the presence of the carbon in the lamp. This embrittlement becomes particularly significant in filaments made of fine tungsten wire having a diameter of less than 0.005 inches and U.S. Pat. No. 4,629,936 suggests various ratios of phosphorus compounds to carbon containing compounds, along with the presence of yellow phosphorus in the lamp, as a possible solution to this problem. The example in this patent employed a mixture of methyl bromide and phosphine, but the amounts of these materials used in the example are not given. The preferred phosphine to methyl bromide mass ratio was said to be from 30/1 to 1/1.
U.S. Pat. No. 4,748,376 discloses that additional problems occur with tungsten halogen lamps when the lamps operate at relatively cool temperatures such that the interior wall surface of the vitreous envelope is less than 200.degree. C. The operation of lamps at these low temperatures is disclosed in this patent as resulting in premature blackening of the glass envelope. Such blackening is believed to be from tungsten transfer from the filament onto the inner envelope surface due to failure of the tungsten cycle. This problem is overcome by using a fill in the envelope which includes bromine, phosphorus, carbon and an inert gas, with a phosphorus to carbon mass ratio in the range of approximately 10/1 to 1/1. Phosphine, methyl bromide, dibromomethane, bromoform and cyanogen (C.sub.2 N.sub.2) are disclosed as being suitable as components of the lamp fill. This patent also discloses that adding too much phosphine to the fill also results in lamp darkening.